By Sarah Khan

The world's third largest river, the Yangtze, suddenly started turning red last week and no one can figure out why!

Usually when a body of water turns red like this it is because of a rapid increase of microscopic algae. Certain species of algae, like dinoflagellates, contain pigments which seep into the water and turn it red. This phenomenon is called a red tide, but it is unlikely that that is the cause of the Yangtze's colour change, as red tide rarely ever happens in fresh water. So, scientists suspect that the Yangtze's red is the cause of pollution.

Last year, another river in China, called the Jian, turned red and it turned out the discolouration was from a couple of workshops that were dumping red dyes into the water. The same cause is not suspected for the Yangtze.

While no one is sure what exactly is the cause of the Yangtze's red colour, there's a good chance that it is just some silt (sand/clay-like substance found in water) or red clay that has been pulled into the water from one of the many steep slopes around the river.

Most residents aren't too worried about the colour change, though, and continue their daily water-based activities. Some people are even collecting bottles of the red water to keep as souvenirs!